How much money is ISIS actually making from looted art?

Fiona Rose-Greenland, Postdoctoral Research Fellow at the University of Chicago, has worked to outline the framework of ISIS’ antiquities trade, and discusses how an "overactive collective imagination" has inflated looting estimates to around 7 billion when it is probably closer to "several million"  ("How much money is ISIS actually making from looted art?" the Conversation 31 May 2016)

The "seven billion" figure comes from here, hardly a 'collective imagination", I would have thought. Neither am I too sure that "only several million" is something the ADCAEA (who seemm to have an unseemly fixation about Syria) should be crowing about.